Several types of digital modulation schemes used in the transmission of radio signals, like Quadrature Amplitude Modulation (QAM) and Quadrature Phase Shift Keying (QPSK) require the use of variable envelope radio frequency signals. The generation and amplification, especially power amplification, of variable envelope radio frequency signals tends to be less efficient and consumes a large percentage of the power in a two way radio. In order to accommodate the amplification of signals having a variable envelope, a power amplifier, which maintains linearity over a wider operating range needs to be used. Most power amplifiers can be tuned to be more efficient at a particular point along the required operating range, often at the expense of efficiency at other points along the operating range.
However some amplifiers can be adapted to operate more efficiently over wider operating ranges than other amplifiers. One such amplifier is an Envelope Elimination and Restoration (EER) amplifier. An EER amplifier generates a signal component representative of a input signal's envelope and couples the signal to the Supply input of a power amplifier. The EER amplifier will also generate a signal component representative of the input signal's phase and couples the signal to the signal input of the power amplifier. As a result, the component of the input signal representative of the signal's phase will have it's amplitude adjusted corresponding to the value of the component of the input signal representative of the signal's envelope, which is coupled to the supply input of the power amplifier.
However, EER amplifiers have seen limited use, because EER amplifiers have significant concerns regarding linearity and bandwidth. While a bandlimited RF signal will generally have baseband signal components in the cartesian domain that are bandlimited, generally the amplitude and phase component of the bandlimited RF signal will not be bandlimited. As a result in order to accommodate non-bandlimited signal components, the bandwidth requirements of the power amplifier system increases greatly. Additionally linearity problems may arise from amplitude modulation to phase modulation distortion due to nulls in the power supply.
The present inventors have recognized that if the EER amplifier can be accommodated with bandlimited approximations of the amplitude and phase/frequency, the substantial bandwidth increases to the system are no longer necessary, and steps can be taken to adjust the approximation algorithm, to incorporate a favorable predistortion, which accommodates the linearity concerns.